The footprint of endolithic algae in shaping the skeletal structure of massive coral skeletons: insights into micro and macro-porosity

Coral skeletons provide habitat for a euendolithic community, forming a green band within the skeleton, where Ostreobium spp. is the dominant group. Euendoliths, actively penetrate live coral skeletons, but how they use and modify skeletal structure is not properly understood. This study explores the microstructural characteristics of skeletal microenvironments through a micro-CT technique that analyzes the "footprint" of the euendolithic community on the porosity of coral skeleton. We compared three Porites species based on the percentage of the relative volume of microporosity, macroporosity, total porosity, and solid volume fraction of CaCO ₃ among three distinct zones within the coral colony: coral tissue, the green band (characterized by eundolithic community) and the bare skeletal region. We found a significant increase in microporosity within the green band, while the opposite occurs for macroporosity that decreased within this zone, for all analyzed species. We describe a model to explain the porosity gradient along the vertical axis for Porites coral colonies, and suggests that within the “green band” microenvironment, the metabolic activity of the community is the responsible for this pattern. Our findings provide insights on the ecological relationship with the coral holobiont: macroerosion mitigation and microporosity filling.